The effect of risk of mortality on the foraging behaviour of animals facedwith time and digestive capacity constraints

This article examines the effect of risk of mortality on the optimal diet o f an animal foraging for two food types. The foods are characterized by dif ferent nutritional or energetic values per unit volume, and the forager has constraints on both the amount of time it has available for foraging and o n the volume of food that can be processed per unit time. Such a situation characterizes the diet choice problem faced by many herbivores. The two foo d types may occur in the same habitat, or they may occur in different habit at patches; in the latter case, they cannot be encountered simultaneously. Unlike earlier analyses of these diet problems, here we consider the risk o f mortality when foraging and allow risk to differ between habitats. Optima l time allocation strategies and the resultant functional responses are cal culated for both one- and two-habitat situations. Mortality risk can substa ntially change the forager's time allocations and, consequently, its functi onal responses. Increasing mortality risk in both habitats proportionally c an increase use of the habitat that has the greater risk. Time allocation o ften responds in a non-monotonic fashion to changes in the density of a par ticular food. As a result, functional responses may decrease with increasin g food abundance over one or more ranges of abundance. Experimental finding s on the response of grasshopper foraging to the risk of spider predation a re compared with the theory. Finally, the possible indirect interactions be tween the food species in this simple food web are discussed.